Human endothelial cells (HEC) and smooth muscle cells (SMC) were incubated with angiotensin I (AI), bradykinin, angiotensin II (AII), tetradecapeptide renin substrate and hog plasma renin substrate to observe angiotensin converting enzyme (ACE), kininase, angiotensinase and renin activities. AI conversion, disappearance of kinin and AII, and generation of angiotensin from renin substrates were measured by bioassay on rat uterus and/or guinea pig ileum. Angiotensin conversion also was studied using (3H)-(10-Leu)-AI and monitoring disappearance of radioactive AI and appearance of (3H)-His-Leu by electrophoresis on cellulose plates. The product of incubation of HEC with (3H)-(8-Phe)-bradykinin was shown to be 3H-Phe-Arg by co-chromatography with authentic Phe-Arg. The molecular weight of ACE was estimated by gel filtration and by sucrose density gradient centrifugation.